Condensation of zinc from its vapor in gaseous mixtures



June 14, 1949. s. RoBsoN 2,473,304 CONDENSATION OF ZINC FROM ITS VAPORIN GASEOUS MIXTURES Filed June 18. 1947 4 Sheets-Sheet 2 n m5 -3 km .3mm x 3. g \v m\\ \\\\\\\\\\.\\R\\\\\\\\\\\\\\\v% .xw w 7 1 a \Q g\\\\\\\\\\\\Q\\\\\\\ Q Q mm x mw v ww w N M June 14,1949; sPRoBS oN Y2,473,304

CONDENSATION OF ZINC 'FR ITS VAPOR IN GASEOUS MIX S Filed June 18, -l9474 Shegts-Sheet 3 Patented June 14, 1949 CONDENSATION OF ZINC FROM ITSVAPOR IN GASEOUS MIXTURES Stanley Robson, Redland, Bristol, England,assignor to The National smelting Company Limited, London, England, aBritish company Application June 18, 1947, Serial No. 755,443

' In Great Britain March 12, 1946 Section 1, PublicLaw 690, August 8,1946 Patent expires March 12, 1966 4 Claims.

This invention relates to an improved method of condensing liquid zincfrom a mixture of zinc vapour with permanent gases. It is adapted todeal with, for instance, the gaseous products evolved when oxidisedzinciferous materials are reduced by carbonaceous reducing agents inexternally heated retorts, electrothermic furnaces or shaft furnaces. Itis an object of this invention to provide a condenser in which most ofthe zinc contained in the incoming gases can be condensed to liquidmetal; in particular, besides ensuring that no large amount of zincvapour escapes condensation, the practice of this invention prevents theformation of any large quantity of zinc dust and dross, consisting ofmetallic zinc with more or less zinc oxide, commonly known as bluepowder.

In retort processes of zinc smelting the main overall reaction OCCUIlIlgis reduction of zinc oxide by carbon to give equal volumes of zincvapour and carbon monoxide, according to the equation,

As a typical instance of retort smelting may be mentioned the process inwhich a briquetted charge of oxidised zinc ores and carbonaceousmaterial is heated in a vertical retort. In this vertical retort processit is customary to admit a certain volume of air or other gas or vapour,e. g.

steam, at the bottom of the retort, so that the gases finally evolvedcontain only 30% to 40% Zinc vapour by volume, the balance being chieflycarbon monoxide but including some nitrogen, hydrogen, and a smallamount of carbon dioxide.

From such gases, in the types of condensers usually employed, it ispossible to condense the greater part of the zinc as liquid metal, butquite a considerable fraction, of the order of -15%, is obtained as bluepowder.

When zinc oxide compounds are reduced by smelting in electric arcfurnaces, the main reaction taking place is the same as in retortsmelting, namely, reduction of zinc oxide by carbon,

When zinc ores are smelted in a blast furnace,

of metallic zinc.

the heat required is provided by burning carbon in a furnace that alsocontains the zinciferous material. Consequently the zinc vapour evolvedis mixed with the products of combustion of the fuel. The gaseousproducts may contain only about 5% of zinc by volume; the carbon dioxidecontent may also be about 5%, the balance consisting chiefly of carbonmonoxide and nitrogen.

Sudden chilling in the conventional types of zinc condenser promotesblue powder formation. In the presence of considerable carbon dioxide,slow cooling promotes the formation of oxidized blue powder by reaction(2). Conflicting. requirements are thus encountered in dealing with agas mixture whose zinc concentration is relatively small and whichcontains gases, such as carbon dioxide, that can oxidize zinc.

It is an object of the present invention to reconcile these conflictingrequirements more especially when the gas to be treated has a relativelysmall concentration of zinc vapour and there are present gases, such ascarbon dioxide, that can oxidise the zinc. With such gases, slow coolingpromotes chemical blue powder formation, by giving opportunity forreaction of zinc vapour with carbon dioxide or other oxidising gasespresent, while rapid cooling, at least in a conventional surfacecondenser, tends'to promote physical blue powder formation by causingforma- -tion of droplets of zinc within the gas phase rather than on thecondenser walls. This dilemma is encountered in dealing with the gaseousproducts resulting from treating zinc ores in a blast furnace.

- spray of molten metal; and in the second stage thetemperature of themetallic shower or spray is less thanthe minimum practicable temperaturefor tapping molten zinc for casting into ingots.

The metal on or in which the zinc from the gas mixture isacondensedint-the ffir'st'stageiof condensation is zinc itself. The metallicshower or spray in the second (and optional third) stage of condensationis lead (in practice having a very small amount of zinc in solution)thefirst stage having a shower or spray of liquid zinc.

without appreciable loss of temperature. Rapid chilling is therebyefiected, heat being abstracted from the gases at a high rate by therain of mcltedzinc; appreciableioxidation of? zinc by 5 carbon dioxideis substantially avoided,- and blue powder is not formed to any greatextent,

The compartment in which the first stage of econdensationis:efiected bya shower of molten :lzinc-is isolated from the second stage compart- Forcarrying out this process, a preferred form ment except for the gastransfer opening, so as of apparatus comprises a stationary condensingunit, partitioned to provide two or three internal chambers, each ofwhich contains mechanical means for producing a continuouseshower .orspray of the liquid metal. prises an inlet for the gases into. th firstof the chambers and a stack outlet from the last'chamher, and thechambers communicate with one another above the levels of themoltenimetal in them.

The spray orshowerof. molten metal through which the gasesarecaused topassmaybe pro- :ducedby a number-cf devices. One methodds "to causearotary :paddle wheel to dipuinto a 511001 of the molten metal. not thecondenser? inuwhich liquid-zinc is'ipresent -all portions of". thepaddle-wheel and its -shaft .:are constructed :of, "or encased in,.zaxmaterial, :suchas graphite or: siliconccarbide, that is: not

:attacked'zbye zinc at the temperature -.at which :the condenserzworks..Therpaddle wheels are enclosed in a box having at one endaan openingtfor: the admission of 'the .zinc vapourr-and gases sandaatath otherendtan'outlet'for the gases out f which'zinchasbeen condensed. "Theboxais: iimade of a steelucasing, the. l-id1of .which isarer-movable,andis :lined throughout'with bricks orla cement that is not? attackedby. liquid: zinc or lead. One or two similarly :linedpartitions.separate the chambers in-which successive condensation stages areeffectedand each partition 'has an opening-to allow the gases-topass-from one chamber to. the next.

The gas inlet and outlet "openings "of each chamber should be so locatedwith reference to -the paddle-wheels that" the gases :are compelled totraversethe-shower =or spray of moltenrnetal, th finalexhaust'stack'being'at the top o'f the last chamber at the end remotefrom' that'at which the gases enter. The paddle-wheel should have only asmall: axialv clearance. at I each end from the sides of the chambenatoensure that' the -'=.shower or spray'of molten metal extendsacross thefull width of the chamber. The outletfor .liquid metal from each chambershouldxbe siturated at a level ensuring adequate immersionof .thepaddle-wheel for showering the molten metal at all times. The-liquidmeta-l outlet above re- =ferred to is either atop hole or a weirseparating -two adjacent condensing chambers both conz-taining the sameliquid :metal I as condensing agent, as in athree-stage processwiththefirst -stage using a-shower of molten zinc and'ithe second and thirdstages using showers of: molten .alead,as hereinafter described.Control.of-heat loss from the chambers :may be .obtained'by placing heatinsulating bricks round them as xrequired, .or 1:.by .cooling the 'basei of .thexcham- .bers by meansso-f water jackets orpipes.

' The mechanical condensing apparatus should be situated as close aspossible to'the'outlet by :which the gases leave the *furnace, to ensure:that thelinc-beafing :gases are lbroughtiinto .-contact with the showeror spray of moltenizinc aas soon as:possible;aterleaving-thetfurnace andi The unit also-ocom- 115111 "toavoid mixing of the zinc and lead bathsof ithexfir-staandrsecond stages. The heat insulation of the "firststage condenser compartment ziis preferablyrsol adjusted that all themolten zinc aintaine'd at about the minimum satisfactory vtappingtemperature, say from 500 to 550 0.; ,whereasthe temperature of themolten lead in the second condensation stage may be kept as xlowsas 420C.

The method of condensing Zinc from a mixi-ture of its vapour withpermanent =ga-sesr by bringing the gases directly' from the producing-unit-into a stationary chamber in-which-ashower or spray of molten:lead-ismaintained -byi me- Within'compartments tchanical means such asamotarypaddle-wheel .is described in application .Ser. .-N0.---535',290of ==Robson and Derhanm. filed May L2, .1944, .now .Patent No.2,464,262. In this-process the-temperature of the lead is. normally betweene500" and/550 Cnand a'very large.-quantity of-lead is employed onaccount of the low 'lsolubility of. zinc :inlead, which 'decreases as"the temperature is lowered with cyclic operation, vinlwhich the lead,lisre-circulated. after cooling to. enable dissolved rzinc to beseparated, -.the .rate of circulation of leadmust usually be fromr100 to200 timesthe rate of extraction of zinc by weight. .Inthe-two-.stageprocess ofthe present 'invention, in which the major part -0f-thetotal zinc extraction is q performed in-the first stagebylmoltenzina thequantityef lead required for-.circulation in the second stage is muchless than is requiredin the :single-stage lead-processof=applicationSer. No. 535,290, and the lead can be kept at a lower 5 'temperature.Full advantage can in fact be taken of the fact that leadhas a lowermelting point than zinc, so that'the;gases'when finally "discharged canhave "been'scrubbedat a temperature-below the melting point'of zinc.

--Scrubbing-the gas with "lead "at a low temperature ensures that zincvapour is almost completely removed'from the gas. 'If, however, the leadleaves'the condenser at a temperature below *the'melting-point of zinc,cooling this lead fur- 'ther brings the con'densed'zinc out of solutionas crystals, which have to-be'remelted to produce liquid zinc. It ispreferable that, on the-one hand, th gas should be scrubbed bylead at atemperature below the melting point of zinc,

- while, on the otherhand, the lead should leave .'-the'condenser at atemperature above the melt- ;in'g-point of zinc.

To obtain-' the greatest benefit in this respect, the leadcondensationmaybe performed in two -stages, making, with the first stage ofcondensation by molten zinc, three stagesin all. The twostage leadcondensation is" performed in two "chambers in' series each providedwith mechanizcal shower producing means and counter-current Show ofmoltenmetal and 'gas, the latter passing "from thefirst to the secon'dofthe'two lead conudensation chambers and-the molten lead flowing from thesecond to the first chamber.

Such a three-stage plant, using molten'zinc ::andlead-inseriesissuitablefcr thetreatment of gas from a blast furnace in which sinteredzinc ore is reduced. In a typical plant of this kind the firstcondensing apparatus, containing a device for producing a shower orspray of molten zinc, is situated as close as possible to the outlet bywhich the gases leave the furnace; rapid chilling is thereby effected,and any considerable condensers, each fitted with apparatus forproducing a shower or spray of molten lead, the last of these condensershaving an opening in the roof for the stack by which the gases finallyleave. Through these two latter condensers molten lead is circulated, incountercurrent with the gas stream; lead is introduced continuously, ata temperature of, say 350 0., into the chamber through which the gasesfinally pass before leaving by the stack, and flows over a weir into thenext compartment, whence it is allowed to flow out continuously, itstemperature then possibly being 500 C. The lead, containing some zinc insolution, is cooled under controlled conditions to separate some of itsdissolved zinc, and is then re-circulated. A wall between the zincspraying chamber and the adjacent lead spraying is of sufficient heightto prevent flow of liquid metal from one compartment to the other, butthere is sufiicient free space above the top of the wall to permit freetravel of gases.

The paddle-wheel referred to can have a variety of forms. It mayconsistof a drum with projecting paddles, which may be shrouded by endflanges. Alternatively it may have'a saw-tooth profile. Furthermore, thedepth of the projections, and the number of them disposed round thecircumference, may be varied. In one case, which may be of importance, anotched, fluted or grooved roller is used; i this case there will bevery numerous indentations or, alternatively regarded, very numerousteeth, all of small size. Whatever type of rotating apparatus isemployed, it is advisable to cool the glands through which the shaft isintroduced into the condenser by means of water. There is serious riskof zinc vapour diffusing to the glands and solidifying there.Arrangements should be made to force a slow stream of gas from theoutside through the glands to ensure that no zinc vapour can reach them.A suitable gas for this purpose is one consisting chiefly of carbonmonoxide, such as the condenser gas after it has been scrubbed andcooled.

The spray-producing device need not necessarily revolve round ahorizontal axis. For instance, one type of device that may be usedcomprises a propeller, or series of cones or cups. or a centrifugalimpeller, rotated by a vertical central shaft introduced through theroof of the chamber.

Another method, particularly convenient for the stage or stages ofcondensation for which lead is used, is to allow a stream of the moltenmetal to fall on to a table which is kept in rapid rotation by avertical shaft.

The process of this invention may also include the stepof condensingresidual zinc from the gases exhausted through the stack in the form ofblue powder and allowing this blue powder to fall into the scrubbingzone of the condenser for recovery as liquid metal.

The accompanying drawings illustrate schematically some typical examplesof zinc condenser assemblies according to the invention, and since themanner of carrying out our improved process, and various modificationsthereof, have already been fully described and the apparatus explainedin general terms, a brief description of each condenser assemblyillustrated will sufiice for its understanding.

The drawings also include figures illustrating typical examples ofpaddle-wheels and other showering devices for molten metal for use incondenser assemblies according to the invention.

In the drawings:

Figs. 1 and 2 respectively are schematical central vertical sections ofa first and second example of a condenser-assembly;

Figs. 3 and 4 are respectively transverse sections of two alternativeexamples of paddle-wheel or rotor structure;

Figs. 5 and 6 show two further examples of paddle-wheel or rotorstructure in transverse section (outline only) Fig. '7 is a centralvertical section of a centrifugal impeller mechanism for showeringmolten metal, with vertical axis;

Fig. 8 is a transverse section on the line 8-8 of Figs. 7 and 9;

Fig. 9 is a view in elevation of the impeller itself.

In Fig. 1, showing a two-stage condenser, I represents a flue by whichthe zinc-bearing gases are conducted from the source, which may, forexample, be a zinc blast furnace, to the condenser, which is dividedinto two compartments, 2 and 3. In compartment 2 is a paddle-wheel 4dipping into a pool of molten zinc 5. In compartment 3 is a similarpaddle-wheel ii dipping into a pool of molten lead I. The floor 8 of thecondenser is constructed of, or lined with, suitable refractory materialthat is not attacked by liquid zinc or lead. The lids 9 and H! of thecompartments are r removable and are similarly constructed. Be-

tween the two compartments there is a baflie ll depending from the roof;beneath it is a bafiie l2 supported from the floor and of sufficientheight to prevent liquid zinc or lead flowing from one compartment tothe other. Through the opening between these baffles the gas passes fromcompartment 2 to compartment 3. At the inlet end of compartment 2 is adepending baffle i3 which compels the incoming gas to pass through anopening I 4 just above the level of the molten zinc. The paddle-wheel 4is rotated in a direction shown by an arrow such that the lower half ismoving towards the opening 14; this ensures that the entering gases areimmediately brought into contact with a spray of molten zinc. Incompartment 3 the paddle-wheel 6 is moving in the opposite direction (asshown by an arrow), so that its lower half is moving towards the gasoutlet I5, which is limited at the top by a depending bafiie it. Thisarrangement ensures that the gases traverse, firstly a spray of moltenzinc in compartment 2, and then a spray of molten lead in compartment 3.

The pool of liquid zinc 5 is connected with an outer well H by anunderflow weir l8. From this well I! the zinc is run off continuously or"re moved at frequent intervals, to maintain the desired level of zincin compartment 2 and thus 7 5 a suitable depth of immersion of thepaddlewhee1 7 -4. Should-the tempera-tureeofi thezinc-in compartment 2 riseabove thedesired-figure,--which--is .generallyabout 500 C.'-to- 550 C1,this canbe corrected by effectin auxiliary cooling in the well l 'Lorby=cireul-ating coolingwaterin a jacket round the-sump. -Shou1d-thetemperature oi -the szinc become too low; this can'be corrected byplacing insulation bricks round compartment 2.

- Lead is continuously circulated through-com- ;partment 3, entering byipipe 28 and-leaving by pipe l9; it takes some zinc into solution-andrises in temperature-during-itspassage. It-is then cooled -undercontrolled conditions 'to separate a-someotits dissolved-zinc and isreturned through the pipe 2 9.

In Fig 2, the first condensing chamber'2l :con- -tains"a. paddle-WheeI-ZZ "which rotates and -=dips intoa pool 230i molten zinc, producing ashower of molten zinc. This efiectsrapid chilling-of the -zinc bearinggasesentering from aflue 24* be- J'neath a baffle 25 to 'be brought intocontact with the shower of molten zinc. The zinc condensed can berun-off under a bafile 26 to an outside well 22', whence it can' be:removed byany convenient means; alternatively, the Zinc can be run offdirectly iroma ta'p hole (not shown) arranged in chamber 2 l. -.Thetemperature of the molten Zinc is maintained at about550 C. If'itbecomes :too cold, insulating bricks may-be placed round chamber 2 I. Ifthe zinc becomes too hot, cooling may convenientlybe efieeted by animmersion water-cooler in well 21. Alternatively cooling may beeiiectedby a water-jacket round the sides and bottom of the sump.

From the z inc sprayingcondenser 2 I the gases then passthrough twocondensers 23 and 29, each fitted with a paddle-wheel and 3|respectively, 'for pr oducing' a-shower or spray of molten lead. Fromchamber 29 the gases leave by a flue 32. 'While theopening to flue 32may be situated directly in the roof of chamber 29, it is found con-"venient to have a'baffle 33 interposed to force the "gas toenterthe'flue near the bottom of chamber 29. Moltenlea'd,aha-temperature of, say, 350 (3., is introduced into chamber 29 by pipe34 and forms a pool BS-into which-the paddle-wheel 3| 'clips. The-leadthen flows over a weir '36 into chamber 28," where it forms a p0ol31,into which "paddle-wheel 3U dips. Thence the lead flows out continuouslythrough pipe 38, its temperature 'then bein 'about 500 C. Thelead,containing some zinc in solution,- is-cooled under controlled conditionsto separatesome of its dissolved zinc "and is'then recirculated back bypipe'34. Arrows indicate the directions of rotation of the paddlewheels.

Between the zinc spraying chamber 2| and the adjacent leadsprayingchamber '28 is a wall '39 of sufficient height to prevent flowof liquid metal 'between chambers2l and 22. The space above "this wall39 up to the roof maybe left open, but it is found preferable to hang abafile 40 from the roof to prevent spray being thrown from onecompartment to another. There is sufficient space hetween bailles -39and40 to permit free flow of gas "from compartment 2! to compartment 28.A *depending bafile 4i -is also.interposed'between chambers 28 and 29.

Some typical examples of paddle-wheels and other rotary devices forshowerin molten metal for use in condensers according to this'inventionwill now be briefly described. 1 I'n Fig. 3,4! isapaddle-wheelcomprisingsa metal drum 42 with-projecting paddles 43, which-mayf'beshrouded 'by end;--flanges-%44; This:--is=suitable onlyfortuse-in-lead. I

:In- Fig. 4-isshown a rotor ldwitha-saw-tooth profile 41. This may-be-constructed-of graphite and carried by a-waterecooled 'metalshaft-48 which extends through the side walls-ofthev eon- "denser. Separating-the-hollow shaft 48 from --direct.contact with-the graphite is-a sleeve4913f insulating cementfembeddedinwhich-are several-ribs-'50'-projecting-irom-the shaft 48. Similarly, the cementis-keyed-to the-g-raphite'by providing recesses5|-in-- the graphitewhicharefilled with cement.

-'Fig.5- shows the profile of arotor: 56- construct- -edof-=graphite,-so shaped as to provide cups viil which serve to pick up the moltenmetal into which it 'dips. y,

Fig. 6 shows the profile ofagraphite rotor 52, notched withnumerousindentations 5-3, or, alternatively regarded, provided withnumerous teeth'M. Thisisshrouded by end flanges 55, also consisting ofgraphite.

(Fig. -7 shows "a "centrifugal "impeller device "which-consists of-a;generally cylindrical rotor 58 --which-is-carriedbya -hollow metal shaft59, verrt-ically mountedand extended through the roof 60 of thecondensing chamber. The rotor 58 may -be' constructed-of graphite orother; suitable refraetory material and is separated from vdirect:contact withthe shaft .59 by a sleeve BI .01? :insulati-ng cement.Ofone piecewith the rotor is an upwardly extending hollow cylinder .62.-of graphite which surrounds the shaft -59, with .a layer of insulatingscement-in between, up to the roof'fifi'of the condensing chamber. Atits lower end the shaft 59 is furnished with splines 63fso *as'to key itto thewinsulating cement. ln the same region there are recesses i l inthe rotor "andthese'are filled with cement, so'that'the -shaft,-sleeveand rotor are effectively keyed 'to- -gether. Theshaft 59 is cooledby'water or oth'er cooling medium introduced through a pipe I 65 whichterminates just. above the bottom ofthe hollow shaft 59. Leaving the.open end 66 0f this 'pipe, the water flows upwards through-the annulus-61 between the pipe-64 andlthe shaft'59.

,On the outside surface of the rotor areitwo similarand diametricallyopposite ledges cutout of -the-cylindrical surface, each, as shown byFigs-7 and 8, extending from apoint 6801' 69 at the top =0f'the rotortoadi-ametricall'y opposite point 18 or H respectively at the bottom ofthe rotor. Thepath followed'on the cylindrical sur- -face;of the rotormaybe described by considering this surface as unrolled-onto a plane;the ledge from 68 to 10 would then bevertical from -68 to 12,approximately circular, forming a quadrant of a circle, from 12 to '13,and horizontal from T3 to 10. The ledges are recessed into the surface,being cut at a somewhat acute angle.

In horizontal projection the ledges taper .from a maximum depth at'68(69) through a mean depth at 14 (15) 'toize'r'o depth at '10 (H). Thedirection. of rotationis shown by an arrow "in .Fig. 8.

The bottom of 'therotor is "situated at Ia level belowthat of the moltenzinc to be held 'in"'tlie condensing chamber 'andthe upper end'fof'thebody of the rotor .is above the molten zinc level. :.A convenient levelfor the molten zinc is "shown by the line 16- 11. g

The rotating.sleeve"62, fits,"with only 'a narrow clearance, inside astationary sleeve I8extending downwards from the. roof fill'of thecondenserlarid into a. circular-trough "119 "out into 'tlie top of therotor. In this trough is maintained liquid zinc, which forms anefiective seal. The upper end of the stationary sleeve 18 is enclosed ina gas seal, comprising a tight fitting box 80 with an opening in the topthrough which the shaft 59 extends, this being fitted with a bush 82. Inthe side of the box 80 there is an opening 83 through which a slowstream of gas is pumped; a suitable gas for this purpose is oneconsisting chiefly of carbon monoxide, such as the condenser gas afterzinc has been removed from it. This gas passes down the annulus betweenthe stationary sleeve 18 and the rotating sleeve 62, becoming heated asit does so, and then bubbles through the metal contained in the circulartrough 19.

I claim:

1. A process for condensing zinc as liquid from a mixture of zinc vapourwith permanent gases, comprising at least two stages of condensation inwhich the gas-zinc vapour mixture is fed with the minimum possible heatloss from the producing unit to a first condensing stage where themixture is brought into intimate contact with a shower of molten zincmetal at a temperature high enough to permit tapping of molten zinc forcasting into ingots, and the gas mixture is thence fed to a secondcondensing stage in which the mixture is brought into intimate contactwith a shower of molten lead which may contain a residual amount of zincin solution and whose temperature is below the minimum practical 10'temperature for tapping molten zinc for casting into ingots.

2. A process for condensing zinc as claimed in claim 1, including athird condensing stage in which the zinc vapour-bearing gas mixture isbrought into intimate contact with a shower of molten lead which maycontain a residual amount of zinc in solution and whose temperature islower than in the second condensing stage.

3. A process for condensing zinc as claimed in claim 2 in which moltenlead for the second condensing stage is provided by the flow of moltenlead from the third condensing stage.

4. A process for condensing zinc as claimed in claim 3 in which moltenlead is introduced into the third condensing stage at a temperature ofabout 350 C. and leaves the second condensing stage at a temperature ofabout 500 C.

STANLEY ROBSON.

REFERENCES CITED 7 The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,871,657 Bunce Aug. 16, 19322,208,586 Kemmer July 23, 1940 2,238,819 Neve Aug. 15, 1941 2,381,405Griswold, Jr. Aug. 7, 1945

